A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
Manufacturing a typical device by a lithographic process typically includes a plurality of cycles of a variety of steps. These steps may include coating the substrate with a photosensitive material (or otherwise applying a photosensitive material to one or more surfaces of the substrate), projecting an image on the photosensitive material, developing the photosensitive material and processing the substrate, which can include covering the substrate in a new layer of material. One of the problems that may be encountered with the lithographic process is that successive layers are not accurately imaged on top of each other so that there is a so-called overlay error. In order to avoid proceeding onto the subsequent steps when an overlay error already exists which would be detrimental to the component's performance, after each cycle the overlay error may be measured. If the overlay error is too large then the most recent layer can be removed and that step repeated before proceeding onto the next step.
In order to reduce overlay error, substrates are generally provided with a plurality of reference marks so that the position of the substrate on a substrate table in a projection apparatus may be measured very precisely prior to the exposure operation. In this way it is possible to improve the accuracy of the exposure operation because the relative positions of the substrate, the previously applied patterned layer and the patterning device in the lithographic apparatus may be determined.
Another problem with multi-cycle lithographic processes is the deformation of the substrate which can occur with the application of particular layers and/or particular patterns. Deformation includes, for example, topographic 3-dimensional deformation, deformation of the reference marks (shape or depth) or variation of layer properties or thicknesses deposited on the substrate. Chemical mechanical polishing (CMP) is notorious for causing deformation of the substrate. With the use of substrates with a diameter of 300 mm or more, it is expected that substrate deformation may become an even more important factor. In order to reduce deformation, it may be desirable to keep the processes as uniform as possible over the whole area of the substrate. Deformation of the substrate may lead to errors in the imaging of the substrate resulting in the need to repeat a particular operation. Also, during the development of a process for a particular component manufactured by lithography, the process may be optimized to minimize, or at least keep within limits, the amount of substrate deformation. The reduction of overlay error or an error as a result of substrate deformation, or at least early detection of one or more of such errors, may lead to improved yield.